This invention relates to serial printers and, more particularly, to control apparatus for serial printers.
Serial printers have been used extensively in recent years in connection with the printing of computer-based output information. One such printer than has enjoyed substantial commercial success is the so-called "daisy-wheel" impact printer. An example of such a printer is disclosed in U.S. application Ser. No. 505,105, now U.S. Pat. No. 3,954,163, filed in the name of Andrew Gabor on Sept. 11, 1974 for HIGH SPEED PRINTER WITH INTERMITTENT PRINT WHEEL AND CARRIAGE MOVEMENT and assigned to the assignee of the present invention.
The usual way of controlling the direction and speed of rotation of a rotatable print member is by using a so-called "dual-mode" servo, i.e. a servo that operates first in a velocity mode and then in a linear, or position, mode. Switching from the velocity to the position mode occurs when the print member is rotated to a predetermined position or "homing region" relative to its desired stopping position. In order to keep track of the actual position of the print member, a means is provided for storing a count indicative of the actual rotational position of the print member. This count is continuously updated by incrementing or decrementing dependent upon the direction of rotation. Means are also included for calculating the difference between such count and a signal indicative of the desired rotational position of the print member. When the difference reaches a predetermined value, e.g. a count of one, the servo switches from the velocity mode to the linear mode.
The manner of incrementing or decrementing the actual position count is normally done by generating increment or decrement pulses as the print member is rotated in one direction or the other, respectively. An example of an apparatus for generating decrement pulses to update a difference count used in a servo system for controlling the direction and speed of movement and a read/write head assembly of a disk drive is disclosed in U.S. Pat. No. 3,663,880 assigned to the assignee of the present invention.
One problem that has arisen in the past in controlling the positioning of a rotatable print member with a dual-mode servo relates to "overshooting" and a return therefrom. By "overshooting" it is meant a rotation of the print member too far and past the desired rotational stopping position. In prior art systems, frequently an overshoot has resulted in the generation of one additional increment or decrement pulse (depending upon the direction of rotation), the effects of which are not cancelled out by a corresponding decrement or increment pulse, respectively, during the return from overshoot. This extra pulse may cause the servo system to re-enter the velocity mode even though the print member may still be within the "homing region."
It would be desirable, therefore, if an increment-decrement apparatus could be provided wherein this extra, unwanted increment or decrement pulse would be inhibited.